[TheMightySpartan]

Hi all,

I have a turbine in a water pipe, a darrius based drag turbine, I have meshed it and got the y+ at the wall approximately 1. Can I use the Transsition SST to solve the flow field as I am a little confused by the FLUENT manual which I have pasted below any insight would be appreciated.

"Note the following limitations:

• The Transition SST model is only applicable to wall-bounded flows. Like all other engineering transition models, the model is not applicable to transition in free shear flows. The model will predict free shear flows as fully turbulent.
• The Transition SST model is not Galilean invariant and should therefore not be applied to surfaces that move relative to the coordinate system for which the velocity field is computed; for such cases the Intermittency Transition model should be used instead.
• The Transition SST model is designed for flows with a defined nonzero freestream velocity (that is, the classical boundary layer situation). It is not suitable for fully developed pipe / channel flows where no freestream is present. For the same reason, it is also not suitable for wall jet flows. For such scenarios, the Intermittency Transition model should be used instead. Note, however, that it might be necessary to adjust the Intermittency Transition model for such flows by modifying the underlying correlations.
• The Transition SST model has not been calibrated in combination with other physical effects that affect the source terms of the turbulence model, such as:
  • buoyancy
  • multiphase turbulence"

https://www.sharcnet.ca/Software/Ans..._overview.html

Many Thanks
TMS

[LuckyTran]

I'mnot familair with this particular model (thanks for copy-pasting the text by the way), but the part that concerns me is here:

Originally Posted by TheMightySpartan

• The Transition SST model is not Galilean invariant and should therefore not be applied to surfaces that move relative to the coordinate system for which the velocity field is computed; for such cases the Intermittency Transition model should be used instead.

Do the surfaces of your turbine blades move in your simulation? That is, are you using a dynamic mesh? You can still use the transition model if you are able to have stationary walls with respect to the mesh by applying a velocity to the walls.

[kam96]

I have the same question, I've found some papers on modeling Vertical Axis Wind turbines using the Transition SST model and moving mesh method, is this a correct approach? since the model isn't Galilean invariant.

[Martin_Sz]

Yes, you can likely use the Transition SST model for your Darrieus turbine simulation in ANSYS FLUENT given the y+ wall value of approximately 1. Here's why:
* Transition SST Applicability: The limitations mentioned in the FLUENT manual you quoted are mostly relevant if you're dealing with:
* Free shear flows (no walls)
* Moving walls relative to the simulation domain
* Fully developed pipe/channel flows (no freestream)
* Buoyancy-driven flows or multiphase flows (e.g., water-air mixtures)
Since your Darrieus turbine is a wall-bounded flow with a well-defined freestream velocity (the water flow in the pipe), the Transition SST model is generally applicable.
* y+ Wall Value: A y+ value around 1 indicates that your mesh is sufficiently refined near the wall to capture the boundary layer effects, which is crucial for transition modeling.
Therefore, using the Transition SST model for your Darrieus turbine simulation seems appropriate based on the information provided.
Cautions and Recommendations:
* While the Transition SST model is a good choice, it's always advisable to consult the ANSYS FLUENT documentation for best practices and potential limitations specific to your turbine geometry and flow conditions.
* Consider running simulations with both the Transition SST model and a simpler model (e.g., Spalart-Allmaras) to compare the results and assess the impact of transition modeling on your specific case.